One of the methods of producing a steel tube is to use a piercing mill to produce a seamless metal tube. The piercing mill includes a plurality of conical rolls disposed at equal intervals around a pass line and a plug disposed on the pass line between the plurality of conical rolls.
The method of producing a seamless metal tube using the piercing mill is as described below. First, a heated round billet is prepared and placed on the pass line. The round billet is pushed in between the plurality of conical rolls by using a pusher disposed in front of the piercing mill. Once the round billet is engaged with the plurality of conical rolls, the round billet is pierced and rolled through the conical rolls with the plug while the billet is spirally rotated to produce a hollow shell.
During piercing-rolling, the plug pierces the round billet. When the plug front end comes out of the rear end of the round billet, a portion of steel on the rear end part of the round billet, with which the plug front end has been in contact until the plug front end comes out, is broken through. The portion of steel broken through may remain as burrs on the inner surface or on the end part of the hollow shell.
The burrs may induce inner surface flaws on the hollow shell during the rolling processes involving an elongator, a sizing mill, and the like, performed after piercing-rolling.
Methods of preventing the formation of such burrs have been disclosed in JP59-148102U, JP62-199201U, and JP7-214112A. In the methods disclosed in these documents, a hole having a predetermined depth is formed in the center at the rear end of the round billet using a punch or gas before piercing-rolling. Then, the piercing-rolling is performed on the round billet in which the hole has been formed. With the hole formed in the center at the rear end of the round billet, excess material that may form burrs can be removed, and consequently the formation of burrs can be suppressed.
Another method different from the aforementioned methods has been disclosed in JP7-214113A. In this method, the piercing-rolling is performed on a round billet having a single-slot or cross-slot groove formed on the rear end face thereof. According to this document, since the single-slot or cross-slot groove is formed on the rear end face, there is less or no excess material in the central region of the end face, so that the formation of burrs can be suppressed.
In sum, according to the methods described in these documents, by scraping the center at the rear end of the billet, excess material that may form burrs can be reduced to suppress the formation of burrs.
In some cases, however, even if any of these methods is employed, burrs are still formed.